Leak detectors have been used for many applications which include detecting the concentration of specific types of gases. However, using the detector to analyze certain types of gases such as refrigerants, and the like, raises safety and environmental concerns. For example, a leak detector can be designed to detect the concentration of refrigerant gas in surrounding ambient air. This application is important, as leaking refrigerant gas can pose a threat to both health and the environment. As used herein, the term gas refers to any gaseous matter and may refer to, e.g., a combination of elemental gasses, and e.g., (ambient atmospheric gas xe2x80x9cairxe2x80x9d).
Previous portable leak detection equipment use Heated Diodes, Negative Corona Discharge, and other techniques. However, problems existed with these technologies. For example, short sensor life and false alarms have been usual problematic results.
Although infrared technology has existed for many years, infrared technology has not been previously used for leak detection applications for many reasons. For example, manufacturing costs would have made the use of infrared technology too expensive to be used as a leak detector.
A xe2x80x9cclosed pathxe2x80x9d type refrigerant monitor has been previously proposed for passing a modulated light beam through a gas to be analyzed and then to an optical detector, all within a closed optical chamber. The detected light is analyzed to provide information about the gas (for example, concentration). The detected light energy may be bandwidth limited such that a specific range of wavelengths is detected to facilitate analysis of particular gasses. However, there are known problems with the xe2x80x9cclosed pathxe2x80x9d monitors. For example, analysis is complicated by very little light energy being absorbed by low gas concentrations, substantial low frequency noise present in the analysis system and optical losses associated with typical optical pathways.
As one solution, modulation of the light beam has been performed at relatively high frequencies such that a reasonably large signal-to-noise ratio is maintained in the detected signal. However, there are problems with these solutions. Broadband light sources use a filament to produce spectral emission and accordingly, electronic modulation thereof at high frequencies is impractical. The filaments require too much time to heat up and cool down for high-frequency modulation. Therefore, mechanical choppers have been used to provide high-frequency modulation of the optical source. Unfortunately, mechanical choppers consume significant energy, decrease instrument reliability and increase the complexity, size and weight of instruments that they are incorporated within.
Other closed path instruments have operated without a mechanical chopper by using more powerful sources, more sensitive detectors and/or long optical paths (to facilitate increased spectral absorption by the gas analyzed) so that a somewhat lower infrared modulation frequency operation may then become possible. However, these solutions increase the size, cost and/or weight of the instruments to which they are applied.
To date there is no known type of leak detector that operates within a one second or less response time, is portable and battery powered and eliminates the problems described above with the prior art.
A primary objective of the invention is to provide an infrared leak detector that can be both portable and battery powered.
A secondary objective of the invention is to provide an infrared leak detector that can both portable and electrical cord powered.
A third objective of the invention is to provide an infrared leak detector that can operate within a one second or less response time.
A fourth objective of the invention is to provide an infrared leak detector having a long sensor life that does not become easily degraded by exposure to large leaks or other reactive gases.
A fifth objective of the invention is to provide an infrared leak detector having accurate sensitivity and accurate selectivity with substantially no false readings.
A preferred embodiment of the hand-held (portable) leak-detector uses Infrared technology to sense when the probe passes a leak. The detector can include an infrared (IR) chamber, an air pump to draw an air type sample such as refrigerant gas into the chamber, a pyroelectric sensor for detecting a selected compound from the sample in the chamber wherein all the components are supported within a portable housing that can be supported in a single hand of a user. The air pump can have a flow rate of approximately 500 cc/min to allow for sampling and finding a small leak without needing for the device to be within approximately 0.635 centimeters (approximately xc2xc inch) of the leak source.
The IR chamber can include an IR emitter for emitting an IR beam, an IR filter in the path of the beam, an IR (pyroelectric) detector such as back to back piezo capacitors, Lithium Tanalate, and the like for receiving the emitted beam and a reaction tube where the air sample interacts with the IR beam.
The IR-based leak-detection does not attempt to generate or process a signal proportional to the actual level or concentration of Refrigerant in the probe air stream, but instead maintains a zero signal for any constant level of Refrigerant in the air, i.e., that is not frustrated by background levels of Refrigerant that may have built up, which thus allows more sensitivity to a small leak. The IR beam used in the detector is NOT chopped or pulsed to generate a continuous signal at the detector.
The lack of IR pulsing results in a lack of signal when the air sample is clean (i.e., no gas of interest to detect. On the other hand, the detector and electronics is designed to settle to a d/c equilibrium voltage (which is the same as the zero level set by the user), whenever there is any constant level of Refrigerant. In other words, the instrument is designed to NOT detect a constant level of Refrigerant. Therefore a signal is generated only when there is an increase in the Refrigerant concentration that changes (rapidly) within the time frame the instrument is designed to detect. The time frame for detecting change is in the order of one second for typical leak detectors, which is a typical time that the sampling probe is moved across a few inches of piping and fittings. This time frame also correlates with a significant amount of air from the leak source entering the chamber, at the approximate 500 cc/min flow rate. The instrument detects the increase in concentration associated with approaching a leak, but ignores the background concentration of Refrigerant that may have built up in a room over longer periods of time (minutes and hours). Additional favorable byproducts of this non-pulsed approach is (1) the elimination of mechanical and or electrical devices to perform the chopping, (2) the battery consumption and (3) the cost of these devices, and (4) elimination of the noise associated with and generated by chopping or pulsing the emitter IR beam, including noise from the imperfections or variations in the chopping, and the noise associated with a significant non-zero signal level.
The device eliminates the extra signal processing required to determine a leak condition by comparing many actual measured levels of concentration. The invention produces a signal only and directly from the sudden rise in concentration at a leak, and avoids processing a continuous measurement signal.
The accumulator within the detector is used as a type of peak-detector and can include a resistor R26 that pulls the op-amp""s inverting input low, from a nearly constant current sinked to Vee that must be supplied from the amplifier. This therefore keeps the amplifier and diode in an active positive (forward) mode. There are several non-obvious, but important benefits to this accumulator. First, the amplifier is not allowed to operate near zero, where the output diode could turn off, and allow the amplifier to swing negative, causing instability and noise. This forced active-forward mode allows the leak detector to function without a pulsed emitter, and with great sensitivity. (IR systems with pulsed emitters do not face this instability issue, since there is always a signal being generated from the chopped IR at the detector.) The accumulator""s output signal decays at a rate determined by C14 and (mostly) R28. This time constant is designed to provide the user with an attention-getting response from even a very small leak.
Another benefit of the forward-active accumulator is that the zero control always operates slightly positive, for any variations in manufacturing and operation, so it can reference (quiet) ground, rather than (far) between Vee and Vcc. This can allow a more precise and stable zeroing, and provides faster zero setting, due to the relatively low resistance to ground.
The Emitter chamber in the detector can be shaped to maximize direction of the IR down the tube (vs. absorption by the wires, header, etc., while at the same time directing the majority of the radiation, that is not initially aligned with the IR tube, to travel down the tube at large angles, with many bounces off the tube surface, so that the optical length is greatly increased. A longer optical length can increase the probability that the IR radiation will interact with a molecule of the gas being detected. This in turn, can increase the signal strength, and decreases the minimum concentration that can be detected.
The detector sensor can be a pyroelectric device that has, in a preferred implementation, matched back-to-back crystals of, for example, Lithium Tantalite. The output of the 2 crystals can tend to cancel each other when at equilibrium, but produce an electrical voltage when the outer element is heated by exposure to IR radiation.
The output of these 2 parallel detectors can be buffered by a JFET. An IR optical filter is just in front of the IR detector, to allow the detector to only xe2x80x9cseexe2x80x9d the narrow band wavelengths selected for maximum, unique interaction with the Refrigerant. An optical filter is typically placed in the beam-line before the IR interaction tube, to eliminate both large and small wavelength radiation from entering the gas interaction area. This eliminates the chance for this excess energy, out of the band of interest, to interact with foreign gases (e.g., water vapor, CO2) such that re-emission into the band of interest can create noise and false signals.
The air pumps, an electrically noise component, can be isolated from the rest of the device by an IC voltage regulator with filter capacitors on both sides. The sensitive analog circuitry can be powered by a regulated DC/DC converter, with an additional LC filter at both outputs.
The whole leak detector can be powered for several hours from a rechargeable battery, or can be powered continuously from an AC/DC supply module that plugs into 115 Vac or 230 Vac.
Many other types of gases other than refrigerant gas R134a can be detected with the detector. And an extendable bendable probe extension can be used.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment which is illustrated schematically in the accompanying drawings.